A capacitor that physically stores an electric charge and a secondary battery that stores energy through electrochemical reaction are among power storage device cells. A capacitor is characterized in that although its energy density is low, its output density is high and hence it can respond to rapid charging or discharging; a secondary battery is characterized in that although its instantaneous force is inferior to that of a capacitor, its energy density is high and hence its sustainability is superior to that of a capacitor. Accordingly, if there can be realized a power storage device cell that is provided with both the instantaneous force of a capacitor and the sustainability of a secondary battery, it can be utilized in various applications such as a hybrid automobile and various kinds of regenerative brakes.
As described above, a capacitor and a secondary battery are different from each other in terms of the mechanism for storing electric power; among capacitors, a capacitor utilizing an electrolyte (referred to as an electric double layer capacitor, a super capacitor, an electrochemical capacitor, or the like; a lithium ion capacitor is also among those capacitors) is provided with polarizable electrodes (a positive electrode and negative electrode) facing each other across a separator, stores electric charges by utilizing an electrostatic capacitance of electric double layers, which are formed on the surfaces of the polarizable electrodes in the electrolyte, and is formed of materials similar to those of a secondary battery.
Thus, the inventors of the present invention paid attention to a lithium ion battery, among secondary batteries, that has an especially high energy density, and proposed a new power storage device cell in which a lithium ion battery and a lithium ion capacitor, which works by sharing common electrolyte with the lithium ion battery, are configured into a single structure by use of a common negative electrode (e.g., refer to Patent Document 1).